gordon mrs bull

52 MRS BULLETIN/AUGUST 2000

IntroductionTransparent, electrically conductive films

have been prepared from a wide variety ofmaterials. These include semiconductingoxides of tin, indium, zinc, and cadmium,and metals such as silver, gold, and titaniumnitride. In this article, the physical proper-ties of these materials are reviewed andcompared.

A figure of merit for a transparent con-ductor may be defined as the ratio of theelectrical conductivity to the optical ab-sorption coefficient of the film. The mate-rials having the highest figures of meritare fluorine-doped zinc oxide and cad-mium stannate. Physical, chemical, andthermal durability; etchability; conduc-tivity; plasma wavelength; work function;thickness; deposition temperature; uni-formity; toxicity; and cost are other factorsthat may also influence the choice oftransparent conducting material for anyparticular application.

Some Applications of Transparent Conductors

Transparent conductors (TCs) have awide variety of uses. Their ability to reflectthermal infrared heat is exploited to makeenergy-conserving windows. These low-emissivity (“low-e”) windows are thelargest area of current use for TCs. Ovenwindows employ TCs to conserve energyand to maintain an outside temperaturethat makes them safe to touch. The electri-cal conductivity of TCs is exploited infront-surface electrodes for solar cells andflat-panel displays (FPDs). Automaticallydimming rear-view mirrors for automobilesand electrically controlled “smart” windowsincorporate a pair of TCs with an electro-chromic (EC) material between them.Electric current is passed through TCs todefrost windows in vehicles and to keepfreezer display cases frost-free. TCs dissi-pate static electricity from the windows on

xerographic copiers. Glass touch-controlpanels are etched from TC layers. TCs canalso be formed into transparent electro-magnetic shields, invisible security circuitson windows, and transparent radio anten-nas built into automobile windows.

It might appear reasonable to ask whichtransparent conducting material is the best.However, this question does not have aunique answer, since different TCs arebest suited for different applications. Also,a given application may constrain themethod of preparation and thereby affect

the choice of material. In the following, wefirst summarize the methods for prepar-ing TCs. Then, we consider the variousmaterials properties that can be importantin choosing a TC. Finally, we show howthese methods and properties lead tochoices of different TCs that are best fordifferent applications.

Processes Used in MakingTransparent Conducting Materials

The properties of a TC layer dependnot only on its chemical composition, butalso on the method used for its prepara-tion. These preparative methods includephysical methods (sputtering, evapora-tion, pulsed laser deposition) and chemi-cal methods (chemical vapor deposition,sol-gel, chemical bath deposition, electro-plating). Some of the innovations in thesedeposition methods are listed in Table I.

Spray pyrolysis was first used commer-cially more than half a century ago to de-posit conductive tin oxide films on heatedglass plates in batch processes. Since the1980s, chemical vapor deposition (CVD) hasbeen widely adopted in the continuousproduction of glass coated with fluorine-doped tin oxide.23 By far the majority of TCfilms are currently produced in this way.Most of this material is used for energy-conserving “low-e” windows in buildings,

Criteria for ChoosingTransparentConductors

Roy G. Gordon

Table I: History of Processes for Making Transparent Conductors.

Materials and Process Reference

Ag by chemical-bath deposition Unknown VenetianSnO2�Sb by spray pyrolysis J.M. Mochel (Corning), 19471

SnO2�Cl by spray pyrolysis H.A. McMaster (Libbey-Owens-Ford), 19472

SnO2�F by spray pyrolysis W.O. Lytle and A.E. Junge (PPG), 19513

In2O3�Sn by spray pyrolysis J.M. Mochel (Corning), 19514

In2O3�Sn by sputtering L. Holland and G. Siddall, 19555

SnO2�Sb by CVD H.F. Dates and J.K. Davis (Corning), 19676

Cd2SnO4 by sputtering A.J. Nozik (American Cyanamid), 19747

Cd2SnO4 by spray pyrolysis A.J. Nozik and G. Haacke (American Cyanamid), 19768

SnO2�F by CVD R.G. Gordon (Harvard), 19799

TiN by CVD S.R. Kurtz and R.G. Gordon (Harvard), 198610

ZnO�In by spray pyrolysis S. Major et al. (Ind. Inst. Tech.), 198411

ZnO�Al by sputtering T. Minami et al. (Kanazawa),198412

ZnO�In by sputtering S.N. Qiu et al. (McGill), 198713

ZnO�B by CVD P.S. Vijayakumar et al. (Arco Solar), 198814

ZnO�Ga by sputtering B.H. Choi et al. (KAIST), 199015

ZnO�F by CVD J. Hu and R.G. Gordon (Harvard), 199116

ZnO�Al by CVD J. Hu and R.G. Gordon (Harvard), 199217

ZnO�Ga by CVD J. Hu and R.G. Gordon (Harvard), 199218

ZnO�In by CVD J. Hu and R.G. Gordon (Harvard), 199319

Zn2SnO4 by sputtering H. Enoki et al. (Tohoku), 199220

ZnSnO3 by sputtering T. Minami et al. (Kanazawa), 199421

Cd2SnO4 by pulsed laser deposition J.M. McGraw et al. (Colorado School of Mines and NREL), 199522

Criteria for Choosing Transparent Conductors

MRS BULLETIN/AUGUST 2000 53

with smaller amounts going into thin-filmphotovoltaics and other applications men-tioned in the first section.

Although indium tin oxide (In2O3�Sn,ITO) was first made by spray pyrolysis,sputtering has become the preferred modefor its production. ITO is mainly usedin FPDs.

Conductive zinc oxide films have beeninvestigated more recently. One applicationin which zinc oxide is used is in photo-voltaics. Because of its potential lower costand easier etchability, zinc oxide may re-place ITO in display applications.

Materials Properties Relevant to Transparent Conductors

A number of physical and chemicalproperties are related to the performanceof a TC in any given application.

Optical and Electrical Performanceof Transparent Conductors

An effective TC should have high elec-trical conductivity combined with low ab-sorption of visible light. Thus an appropriatequantitative measure of the performanceof TCs is the ratio of the electrical conduc-tivity � to the visible absorption coeffi-cient �,

(1)

in which Rs is the sheet resistance in ohmsper square, T is the total visible transmis-sion, and R is the total visible reflectance.Thus �/� is a figure of merit for ratingTCs.24 A larger value of �/� indicates bet-ter performance of the TC.

Figures of merit for some TCs are givenin Table II. The values given are for thebest samples that we have prepared in ourlaboratory by CVD at atmospheric pressure,except for the indium oxide value, whichis the best that we have measured for acommercially available film, and the cad-mium stannate values, which we have takenfrom the literature.25

The results in Table II show that fluorine-doped zinc oxide and cadmium stannatehave the best figures of merit of these TCs.If the electrical and optical properties of aTC were independent of film thickness,then the figure of merit �/� would not de-pend on film thickness, unlike other figuresof merit that have been proposed.26 In fact,“bulk” properties of TCs, such as � and �,do depend somewhat on film thickness.For example, they depend on crystallitegrain size, which usually increases with filmthickness. The figure of merit thereforegenerally increases with film thickness. Thefilm thicknesses of the samples reported inTable II were chosen to be typical of those

�� Rs ln�T � R��1

needed for low-resistance applicationssuch as solar cells.

The results in Table II show that fluorinedoping gives superior performance com-pared with metallic dopants, in both zincoxide and tin oxide. A theoretical under-standing of this advantage of fluorine canbe obtained by considering that the con-duction band of oxide semiconductors isderived mainly from metal orbitals. If ametal dopant is used, it is electrically ac-tive when it substitutes for the primarymetal (such as zinc or tin). The conductionband thus receives a strong perturbationfrom each metal dopant, the scattering ofconduction electrons is enhanced, and themobility and conductivity are decreased. Incontrast, when fluorine substitutes for oxy-gen, the electronic perturbation is largelyconfined to the filled valence band, andthe scattering of conduction electrons isminimized.

A theoretical upper limit to the figure ofmerit may be estimated from the transporttheory of electrons in metals27 given by

(2)

where �0 is the permittivity of free space,c is the speed of light in vacuum, n is therefractive index of the film, m* is the effec-tive mass of the conduction electrons, � isthe mobility, � is a visible wavelength oflight, and e is the electronic charge. The re-fractive index of TCs is close to 2.0 in thevisible region, thus the highest figure ofmerit will be obtained from the materialwith the highest product of mobility andeffective mass. For zinc oxide,28 tin oxide,29

and cadmium stannate,30 m* is close to 0.3 m,where m is the free-electron mass. Thusmost of the variation in the figure of meritis due to differences in mobility. Note thatthe free-electron concentration does notenter into the figure of merit.

�� 4 2�0c3n�m*��2��2e�2,

The electron mobility is determinedby the electron-scattering mechanismsthat operate in the material. First of all,some scattering mechanisms, such as scat-tering of electrons by phonons, are presentin pure single crystals. In tin oxide29 andzinc oxide,31 these scattering mechanismslead to mobilities of about 250 cm2 V�1 s�1

at low doping levels, typically around1016 cm�3. Practical TCs need much higherdoping levels, usually 1020 cm�3, inorder to operate at reasonable thicknesses.For these high doping levels, scattering bythe ionized dopant atoms becomes an-other important mechanism that alonelimits the mobility to less than about90 cm2 V�1 s�1.33 In the presence of boththese scattering mechanisms, the mobilityis limited to the value (250�1 � 90�1)�1 �66 cm2 V�1 s�1. This maximum mobility islowered still further by other scatteringmechanisms such as grain-boundary scat-tering, present in polycrystalline thinfilms. The best TC films, ZnO�F andCd2SnO4, have been prepared with mobili-ties in the range of 50–60 cm2 V�1 s�1,closely approaching the theoretical upperlimit.

Electrical ConductivityIn some applications of TCs, it is critical

that the TC be as thin as possible. For ex-ample, in high-resolution displays, the re-quired etched patterns in the TC createheight variations in the device. To keep thetopography as smooth as possible, the thin-nest possible TC is desired. In this case,the important material parameter is theconductivity �. The conductivity increaseswith the product of the concentration offree electrons and the mobility. For metalssuch as silver and titanium nitride, the free-electron concentration is fixed by thestructure and electronic properties of thesolid. For wide-bandgap semiconductors,

Table II: Figures of Merit �/� for Some Transparent Conductors.

VisibleAbsorption

Sheet Resistance Coefficient Figure of MeritMaterial (�/�) � (��1)

ZnO�F 5 0.03 7Cd2SnO4 7.2 0.02 7ZnO�Al 3.8 0.05 5In2O3�Sn 6 0.04 4SnO2�F 8 0.04 3ZnO�Ga 3 0.12 3ZnO�B 8 0.06 2SnO2�Sb 20 0.12 0.4ZnO�In 20 0.20 0.2



the free-electron concentration is deter-mined by the maximum number of elec-tronically active dopant atoms that can beplaced in the lattice. Attempts to place alarger number of dopant atoms in the lat-tice simply produce neutral defects thatdecrease the mobility. The maximum ob-tainable electron concentration and themaximum conductivity in TCs generally arefound to increase in the following order:ZnO�F � SnO2�F � ZnO�Al � In2O3�Sn �TiN � Ag.

Plasma FrequencyThe plasma frequency for the conduc-

tion electrons in a TC divides the opticalproperties. At frequencies higher than theplasma frequency, the electrons cannot re-spond, and the material behaves as a trans-parent dielectric. At frequencies below theplasma frequency, the TC reflects and ab-sorbs incident radiation. For most TC ma-terials, the plasma frequency falls in thenear-infrared part of the spectrum, andthe visible region is in the higher, trans-parent frequency range. The plasma fre-quency increases approximately with thesquare root of the conduction-electronconcentration. The maximum obtainableelectron concentration and the plasma fre-quency of TCs generally increase in thesame order as the resistivity, as shown inTable III. The corresponding plasma wave-lengths decrease from about 2 �m forZnO�F to 0.7 �m (red light) for TiN(Table III).

Work FunctionThe work function of a TC is defined as

the minimum energy required to removean electron from the conduction band tothe vacuum. Some work functions meas-ured for TCs are collected in Table IV.

Thermal Stability of Transparent Conductors

TCs will generally increase in resistanceif heated to a high enough temperature fora long enough time. For example, some ofthe TCs were tested in my laboratory byheating them in air for 10 min at succes-sively higher temperatures. Table V gives,as “stability temperatures,” the tempera-ture range within which no increase of10% in sheet resistance was noted.

In each case, the TC remained stableto temperatures slightly above the opti-mized deposition temperature. The high-temperature stability of tin oxide filmsallows coated glass to be reheated in orderto strengthen it by tempering. The thermalstability of tin oxide films is currentlylimited more by the softening of glass sub-strates than by any thermal decompositionof the SnO2�F film.

Cadmium stannate is deposited by sput-tering at low temperatures in amorphousform. When annealed at high tempera-tures, it crystallizes into a crystalline formthat is stable up to at least 1100�C.34

Minimum Deposition TemperatureWhen TCs are deposited onto a substrate,

the temperature of the substrate generallymust be maintained at a sufficiently hightemperature in order to develop the re-quired properties in the TC. The requiredtemperatures usually increase in the fol-lowing order: Ag or ITO � ZnO � SnO2 �Cd2SnO4. Thus, silver or ITO may be pre-ferred for deposition on thermally sensitivesubstrates, such as plastic, while cadmiumstannate requires very refractory substratesto develop its best properties.

Diffusion Barriers betweenTransparent Conductors and Sodium-Containing Glass Substrates

When TCs are deposited on sodium-containing glass, such as soda-lime glass,sodium can diffuse into the TC and increaseits resistance. This effect is particularlynoticeable for tin oxide, because sodiumdiffuses rapidly at the high substrate tem-peratures (often 550�C) used for its depo-sition. It is common to deposit a barrierlayer on the glass prior to the depositionof tin oxide. Silica is most commonly usedas the barrier layer between soda-lime glassand tin oxide, even though silica is onlypartially effective in blocking the transportof sodium. The silica layer usually serves asecond purpose, that of eliminating the in-

Table III: Approximate Minimum Resistivities and Plasma Wavelengths for Some Transparent Conductors.

Resistivity Plasma WavelengthMaterial (�� cm) (�m)

Ag 1.6 0.4TiN 20 0.7In2O3�Sn 100 1.0Cd2SnO4 130 1.3ZnO�Al 150 1.3SnO2�F 200 1.6ZnO�F 400 2.0

Table IV:Work Functions of Some Transparent Conductors.

Work Function Electron ConcentrationMaterial (eV) (cm�3)

ZnO�F 4.2 2 1020

ZnO 4.5 7 1019

In2O3�Sn 4.8 1020

SnO2�F 4.9 4 1020

ZnSnO3 5.3 6 1019

Taken from Reference 33.

Table V:Thermal Stability of Some Transparent Conductors.

Deposition StabilityTemperature Temperature

Material (�C) (�C)

LPCVD ZnO�B 200 �250APCVD ZnO�F 450 �500APCVD SnO2�F 650 �700



terference colors that would otherwise beshown by the TC film.35 Alumina is a muchmore complete barrier against diffusion ofsodium.36

Etching Patterns in TCsFor some applications of TCs, such as

displays, heaters, or antennas, parts of theTC must be removed. Table VI lists somechemicals that can be used to etch TCs.Zinc oxide is the easiest material to etch,tin oxide is the most difficult, and indiumoxide is intermediate in etching difficulty.Series-connected thin-film solar cells alsoneed to remove TCs along patterns of lines.This removal is usually carried out by laserablation.

Chemical DurabilityThe ability of a TC to withstand corrosive

chemical environments is inversely relatedto its ease of etching. Tin oxide is the mostresistant, while zinc oxide is readily attackedby acids or bases. Silver is tarnished by airand moisture and can be used only in ap-plications that are hermetically sealed.

Stability in Hydrogen PlasmasIn forming amorphous-silicon solar cells

on TC superstrates, the TC is exposed to aplasma containing hydrogen atoms. Theseplasma conditions rather easily reduce tinoxide, causing an increase in the optical ab-sorption by the tin oxide. Zinc oxide is muchmore resistant to hydrogen-plasma reduc-tion and may be preferred for applicationssuch as amorphous-silicon solar cells.37

Mechanical Hardness of TCsThe mechanical durability of TCs is re-

lated to the hardness of the crystals fromwhich they are formed. Their hardness val-ues may be ranked using the Mohs scale,in which higher values represent hardermaterials.38 Table VII shows that titaniumnitride and tin oxide are even harder thanglass and can be used in applications thatare exposed to contact. Zinc oxide is readilyscratched, but can be handled with care.Thin silver films are so fragile that theycannot be touched and can be used onlywhen coated with protective layers.

Production CostsThe costs of producing a transparent

conducting material depend on the costof the raw materials and the processingof it into a thin layer. The cost of the rawmaterials generally increases in this order:Cd � Zn � Ti � Sn � Ag � In. Indium isa rare and expensive element that is ob-tained as a byproduct of the mining of oresfor their content of other metals such as zincand lead. There are no “indium mines” be-cause its concentration in minerals is too

low to allow economic extraction only forthe value of the indium. Thus the supplyof indium cannot be increased significantlywithout a large increase in price sufficientto make “indium mines” profitable.

The costs of the deposition methodstypically increase in the following order:atmospheric-pressure CVD � vacuumevaporation � magnetron sputtering �low-pressure CVD � sol-gel � pulsed laserdeposition. This ranking was estimated byconsidering the lowest-cost product madeby each process. The speed of the processis very important in the cost. Atmospheric-pressure CVD, vacuum evaporation, andmagnetron sputtering have high deposition

rates and have been scaled up to large areas.Low-pressure CVD has higher equipmentcosts than atmospheric-pressure CVD. Sol-gel suffers from slow drying and reheatingsteps, while pulsed laser deposition is onlysuitable for small areas. This ranking cangive only a rough comparison of productioncosts because many other factors enter intoa full economic analysis, including the pro-duction volume and the tolerances for varia-tions in the properties of the product.

ToxicitySome of the elements used in TCs are

toxic. This increases the cost of processingthem because of the need to protect work-ers and prevent the escape of toxic ma-terials into the environment. Additionalencapsulation during use may be needed,as well as a provision for recycling at theend of the product’s lifetime. Toxicity ofthe elements generally increases in the fol-lowing order: Zn � Sn � In � Ag � Cd.Cadmium compounds are carcinogensand thus are heavily regulated and evenprohibited from some applications.

Choice of Transparent Conducting Oxides

It is apparent from the diversity of ap-plications for TCs that no one material ismost suitable for all uses. Depending onwhich material property is of most impor-tance, different choices are made. Table VIIIsummarizes some of the most importantcriteria that may influence the choice of atransparent conducting material.

Examples of Applications and the TCs Chosen for Them

We will now see how these criteria applyto a number of applications in which TCsare used.

Table VIII: Choice of Transparent Conductors.

Property Material

Highest transparency ZnO:F, Cd2SnO4

Highest conductivity In2O3�SnLowest plasma frequency SnO2�F, ZnO�FHighest plasma frequency Ag, TiN, In2O3�SnHighest work function, best contact to p-Si SnO2�F, ZnSnO3

Lowest work function, best contact to n-Si ZnO�FBest thermal stability SnO2�F, TiN, Cd2SnO4

Best mechanical durability TiN, SnO2�FBest chemical durability SnO2�FEasiest to etch ZnO�F, TiNBest resistance to H plasmas ZnO�FLowest deposition temperature In2O3�Sn, ZnO�B, AgLeast toxic ZnO�F, SnO2�FLowest cost SnO2�F

Table VI: Etchants for TransparentConductors.

Material Etchant

ZnO Dilute acidsZnO Ammonium chlorideTiN H2O2 � NH3

In2O3 HCl � HNO3 or FeCl3SnO2 Zn � HClSnO2 CrCl2

Table VII: Hardness of SomeTransparent Conductors.

Material Mohs Hardness

TiN 9SnO2 6.5Soda-lime glass 6In2O3 �5ZnO 4Ag low



Low-Emissivity Windows in BuildingsTCs on window glass improve the en-

ergy efficiency of the window because thefree electrons reflect infrared radiation forwavelengths longer than the plasma wave-length. The effect is similar to that of thesilver coating in a Thermos bottle. In coldclimates, the plasma wavelength shouldbe fairly long, about 2 �m, so that most ofthe solar spectrum is transmitted into heatinside the building. Fluorine-doped tinoxide is the best material for this purposebecause it combines a suitable plasmawavelength with excellent durability andlow cost. Billions of square feet of TC-coated window glass have been installedin buildings around the world.

In hot climates, a short plasma wave-length, �1 �m, is desirable, so that thenear-infrared portion of incident sunlightcan be reflected out of the building. Themetals silver and titanium nitride havesufficiently short plasma wavelengths forthis application. Silver is widely used forthis application despite its poor durability;it is sealed inside double-glazed panes forprotection from air and moisture. Titaniumnitride is much more durable and can beused on exposed surfaces, even on single-glazed windows. The reflective gold colorof TiN-coated glass can frequently be seenon large office buildings, but it is not popu-lar for residential windows.

Solar CellsThe front surfaces of solar cells are cov-

ered by transparent electrodes. In single-crystal silicon cells, a highly doped layerof the silicon itself serves as the front elec-trode. In thin-film cells, a TC layer servesas the front electrode. Cadmium tellurideand some amorphous-silicon solar cellsare grown on a SnO2�F-covered glass super-strate. Thermal stability and low cost arethe primary factors in this choice. The highwork function of SnO2�F is also helpful inmaking low-resistance electrical contact tothe p-type amorphous-silicon layer. Otheramorphous-silicon cells are grown on flexi-ble steel or plastic substrates; in this case,the top TC must be deposited at low tem-perature on thermally sensitive cells. ITOor ZnO is chosen for this purpose becauseboth compounds can be deposited suc-cessfully at low temperatures (typically�200�C).

Flat-Panel DisplaysThe many different styles of FPDs all

use TCs as a front electrode. Etchability isa very important consideration in formingpatterns in the TC electrode. The easieretchability of ITO has favored its use overtin oxide, which is more difficult to etch.The low deposition temperature of ITO is

also a factor for color displays in whichthe TC is deposited over thermally sensi-tive organic dyes. Low resistance is anotherfactor favoring ITO in very finely patterneddisplays, since the ITO layer can be madevery thin, thus the etched topography re-mains fairly smooth. ZnO is lower in costand easier to etch than ITO is, so ZnO mayreplace ITO in some future displays.

Electrochromic Mirrors and WindowsAutomatically dimming rear-view mir-

rors are now installed in millions of auto-mobiles. They include a pair of SnO2�F-coated electrodes with an electrochemi-cally active organic gel between them. Themain considerations are chemical inertness,high transparency, and low cost. “Smart”windows with electrically controllabletransmission are just entering the market-place. Tin oxide appears to be the materialof choice, for the same reasons that it ischosen for electrochromic mirrors.

Defrosting WindowsFreezers in supermarkets pass electric

current through TCs on their display win-dows in order to prevent moisture in theair from condensing on them and obscur-ing the view. Low cost and durability arethe main factors that have led to the choiceof tin oxide for this application.

Defrosting windows in airplanes wasthe first application of TCs, permitting high-altitude bombing during World War II. Thediscovery of TCs was kept secret until afterthe war. Originally tin oxide was used, butnow ITO has replaced it in modern cock-pits because its lower resistance permitsdefrosting larger window areas with rela-tively low voltage (24 V). Some automobilewindshields use silver or silver-copperalloy TCs for electrical defrosting becausethe 12-V systems in automobiles requirevery low resistance, combined with thelegal requirement of a minimum transmis-sion of 70%. The metal layers are protectedin the windshield by laminating them be-tween two sheets of glass.

Oven WindowsTin oxide coatings are placed on oven

windows to improve their safety by low-ering the outside temperature of the glassto safe levels. This permits the use of win-dows even in self-cleaning ovens that reachvery high temperatures. The tin oxide coat-ing also improves the energy efficiency ofthe ovens. The main criteria for this choiceof material are high temperature stability,chemical and mechanical durability, andlow cost.

Some transparent laboratory ovens areconstructed entirely of TC-coated glass,

which also serves as the electrical resistorfor heating the oven.

Static DissipationTCs are placed on glass to dissipate

static charges that can develop on xero-graphic copiers, television tubes, and CRTcomputer displays. Only relatively highresistances (�1 k�/�) are needed, so themain concern is mechanical and chemicaldurability. Tin oxide is the material of choicefor these applications.

Touch-Panel ControlsTouch-sensitive control panels, such as

those found on appliances, elevator con-trols, and ATM screens, are formed frometched TCs on glass. They sense the pres-ence of a finger either by direct contact orcapacitively through the glass. The dura-bility and low cost of tin oxide make it agood choice for these applications.

Electromagnetic ShieldingIt is apparently possible to eavesdrop

on computers and communications by de-tecting electromagnetic signals passingthrough windows. These stray signals canbe blocked by TCs with low sheet resis-tance. Silver and ITO are the best materialsfor this purpose.

Invisible Security CircuitsTC-coated glass can be used as part of

invisible security circuits for windows oron glass over valuable works of art. Someprotection from fading by UV light is alsoprovided by the TC. Any TC (except forcolored TiN) could be used. Silver/ZnOmultilayers provide the best UV protection.

Improving the Durability of GlassSome tin oxide coatings are used solely

to take advantage of tin oxide’s extraordi-nary durability and have nothing to dowith its electrical conductivity. Tin oxidecoatings are used on the windows of bar-code readers to improve their abrasion re-sistance. Hydrofluoric acid etches glass,but does not affect tin oxide. Some vandalshave used hydrogen-fluoride etching kits(designed to etch identifying marks on auto-mobile windows) to etch slogans on win-dows. Tin oxide coatings are used to protectwindows from these attacks.

ConclusionsTransparent conductors have many

applications. There is no one TC that isbest for all applications. Fluorine-dopedtin oxide is the most widely used TC, whiletin-doped indium oxide (ITO) remains pre-ferred for flat-panel displays. Zinc oxidehas potential for use in more efficient andless expensive solar cells. All of these com-



monly used transparent conducting mate-rials and their production methods haveadvantages and disadvantages that mustbe carefully weighed for each new appli-cation. The information in this article mayhelp in choosing the most appropriate trans-parent conducting material for a new use.

AcknowledgmentsThis work was supported in part by the

National Renewable Energy Laboratoryunder a subcontract for the U.S. Departmentof Energy.

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17. J. Hu and R.G. Gordon, J. Appl. Phys. 71(1992) p. 880.18. J. Hu and R.G. Gordon, J. Appl. Phys. 72(1992) p. 5381.19. J. Hu and R.G. Gordon, in MicrocrystallineSemiconductors: Materials Science & Devices, ed-ited by P.M Fauchet, C.C. Tsai, L.T. Canham,I. Shimizu, and Y. Aoyagi (Mater. Res. Soc.Symp. Proc. 283, Pittsburgh, 1993) p. 891.20. H. Enoki, T. Nakayama, and J. Echigoya,Phys. Status Solidi A 129 (1992) p. 181.21. T. Minami, H. Sonohara, S. Takata, and H.Sato, Jpn. J. Appl. Phys., Part 2: Lett. 33 (1994)p. L1693.22. J.M. McGraw, P.A. Parilla, D.L. Schulz,J. Alleman, X. Wu, W.P. Mulligan, D.S. Ginley,and T.J. Coutts, in Film Synthesis and GrowthUsing Energetic Beams, edited by H.A. Atwater,J.T. Dickinson, D.H. Lowndes, and A. Polman(Mater. Res. Soc. Symp. Proc. 388, Pittsburgh,1995) p. 51. 23. P.F. Gerhardinger and R.J. McCurdy, in ThinFilms for Photovoltaic and Related Device Applica-tions, edited by D. Ginley, A. Catalano, H.W.Schock, C. Eberspacher, T.M. Peterson, and T.Wada (Mater. Res. Soc. Symp. Proc. 426, Pitts-burgh, 1996) p. 399. 24. R.G. Gordon, in Thin Films for Photovoltaicand Related Device Applications, edited by D. Ginley, A. Catalano, H.W. Schock, C. Eberspacher,T.M. Peterson, and T. Wada (Mater. Res. Soc.Symp. Proc. 426, Pittsburgh, 1996) p. 419; a simi-

lar expression was proposed by V.K. Jain andA.P. Kulshreshtha, Sol. Energy Mater. 4 (1981)p. 151, without taking into account the reflection.25. T.J. Coutts, X. Wu, W.P. Mulligan, and J.M.Webb, J. Electron. Mater. 25 (1996) p. 935.26. G. Haacke, J. Appl. Phys. 47 (9) (1976)p. 4086.27. For example, see S. Nudelman and S.S. Mitra,Optical Properties of Solids, Chap. 3 (PlenumPress, New York, 1969).28. A.R. Hutson, J. Appl. Phys. 32 (1961) p. 2287.29. K.J. Button, C.G. Fonstad, and W. Dreybrodt,Phys. Rev. B 4 (1971) p. 4539.30. W.P. Mulligan, PhD thesis, Colorado Schoolof Mines, 1997.31. G. Bogner, J. Phys. Chem. Solids 19 (1961)p. 235.32. J.R. Bellingham, W.A. Phillips, and C.J.Adkins, J. Mater. Sci. Lett. 11 (5) (1992) p. 263.33. T. Minami (private communication).34. M. Hassanein, J. Chem. U.A.R. 9 (1966) p. 275.35. R.G. Gordon, U.S. Patent No. 4,187,336(1980); U.S. Patent No. 4,419,386 (1983).36. J.D. Chapple-Sokol, PhD thesis, HarvardUniversity, 1988.37. H.N. Wanka, E. Lotter, and M.B. Shubert, inAmorphous Silicon Technology–1994, edited byE.A. Schiff, M. Hack, A. Madan, M. Powell, andA. Matsuda (Mater. Res. Soc. Symp. Proc. 336,Pittburgh, 1994) p. 657.38. D.R. Lide, ed., Handbook of Chemistry andPhysics (CRC Press, Boca Raton, FL, 1999) p. 12.

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